OF all the projects that have excited the ridicule of the unimaginative of times gone by, perhaps none has appeared more exceedingly funny and chimerical than that of producing at will, by mechanism operated by heat, a freezing cold, and that without the use of ice, or any previously congealed substance, and without regard to atmospheric temperature.

In these days of rapid development of the mechanic arts, it seems hazardous to assert impossibility of any mechanical problem involving the substantial amelioration of man's condition. The manifest need of an improvement seems to be but the condition of its realization and development; sooner or later appears the embryo invention destined to be the theme of long study and continual modification, the perfected product often bearing little or no resemblance to the crude prototype that may have first embodied an idea fraught with lasting good to man. The conception once concretely realized, its beneficent results become a part of the common capital of the race, making possible still further advances in our material well-being.

While the progenitors of the race seem early to have discovered the means of producing heat artificially, for their rude arts and for their bodily comfort, it is not probable that the means of obtaining artificial cold could ever have seemed to primitive man a pressing need. Civilization is a multiplying of needs, and nothing connected with man's development seems more clear than that the adoption of artificial protection from the elements, conducing directly as it has to a material modification of Nature's means of protecting the body and providing for its wants, has not only led to the demand for readily available means of producing artificial heat, but for the means of artificial refrigeration as well.

Since the experiments of Professor Twining thirty years ago, with sulphuric ether, the problem of producing artificial cold has been attacked by many, but the basis of the more important and successful systems employed has been, as in Twining's experiments, the volatilization of a liquid in vacuo, by means of a gas-pump. Of the various substances available in nature for this purpose, ether and ammonia have received the most attention. Various other liquids have also been used, such as sulphide of carbon, methylic ether, chloride of methyl, chymogene, etc., and latterly sulphurous acid as used in the famous Pictet system. Compressed air has also been employed, but the mechanical labor required by this system is too costly to allow it to compete with what may be termed the volatilizing systems.

The object sought has been the most economical method of employing those substances that are capable of producing the greatest degree of cold. But a difficulty is encountered in the high pressures of the gases produced in the pump, as there is no evading the physical fact that the cold-producing power of a gas is a concomitant of its tension, or pressure varying directly therewith. Thus, ammonia, with a pressure at rest of eight atmospheres, and at work of twelve to twenty atmospheres according to the temperature, is an excellent refrigerant, but the use of a gas with such a high pressure is attended with obvious drawbacks. At the other end of the scale is ether, which is manageable at a low pressure, viz., zero at rest, and ten to fifteen pounds per square inch at work; but this advantage has its corresponding drawback, in accordance with the law above mentioned, i. e., a comparatively low refrigerating power. It is, moreover, inflammable, and, in contact with any of the lubricants used on the pumppiston, there results an unintended product of soap, which, coating the parts of the mechanism, obstructs the passage of the latent heat from the circulating medium employed for freezing. Midway between these two agents, as regards its pressure, is sulphurous acid. This gives a high degree of cold, its pressure at work being three and one half to six atmospheres, and a little over two and one half atmospheres at rest. Aside from its rather high pressure, a serious objection to its use is the liability to corrosion of the parts on contact of the liquid with moisture, sulphuric acid being thereby produced, which rapidly wears away the more important parts of the mechanism employed.

The various defects enumerated, and others incident to the use of other agents not here particularized, viz., liability to explosion, inflammability, indifferent refrigerating capacity, high vacuum, high pressure involving rapid wear and tear and danger in use, and other more or less serious drawbacks, have made the attainment of a still better system than the best of those referred to imperative. The great desideratum, it will be seen, has been a process admitting of using some of the better cold-producing agents without the dangers or annoyances due to the high tensions of their gases, or to other peculiarities of their composition. The discovery of a method by which this object could be attained is due to the genius of the late C. M. Tessié du Motay.

This eminent French chemist, acting on the suggestion of one of his associates, M. Étienne Gillet, a gentleman who had made a close study of artificial ice-making, sought to combine two or more liquids which should have the property, in combination, of mutually neutraizing the defective features they exhibited when used separately, and which should at the same time retain their desirable qualities. He instituted experiments, in conjunction with M. Auguste Rossi, which resulted in the discovery that ether, when combined with sulphurous acid, furnished a compound absolutely free from any of the defects that had previously hindered successful working. The inflammability of ether was nullified by the sulphurous acid; a perfect lubricant was obtained, and the substance had no corrosive action on the metals employed. But the most interesting feature developed by the experiment was that the ether was found to have the power of absorbing a large proportion of the gas of the sulphurous acid. This is the characteristic feature of the binary absorption system, as Du Motay termed his process. The ether, by absorbing the gas of the other constituent liquid, reduces the mechanical problem to that of liquefying a gas having a pressure not approximating that of sulphurous acid, viz., fifty to eighty pounds or more per square inch, but barely more than that of ether itself, viz., twenty pounds. The pressure of the compound at rest, like that of ether, is nil. In other words, the ether is found to have accomplished the greater part of the work, and a law of nature governing the action of certain chemicals in combination is availed of to reduce the mechanical labor of liquefaction to a minimum.

Since the death of Du Motay, which occurred very soon after his discovery, his associates, MM. Auguste Rossi and Leonard F. Beckwith, have continued the experiments under the Du Motay patents, with various other compounds, and have accomplished the hitherto unheard-of result of liquefying ammonia gas in the pump at a pressure of thirty-five pounds per square inch. This is accomplished by combining it with glycerine, a non-volatile, which gives up the ammonia gas in the vacuum-pump, but, when it has reached a certain tension, seizes it, so to speak, and renders it liquefiable at a fraction of its ordinary pressure.

There are various other compounds capable of giving the same results — an intense freezing power at a greatly diminished pressure, and the peculiarities of various industries employing mechanical refrigerants can thus be consulted and met by the use of whatever compound is found best adapted thereto.

There are certain general features common to all the systems employing a liquid volatilizable in the vacuum-pump, but the peculiar features of the binary absorption process admit of such a simplifying of the mechanical appliances employed as to materially distinguish their construction from that of other systems.

The freezing agent, ethylo-sulphurous dioxide, or glycerine and ammonia, or whatever be the compound employed, is placed within the "refrigerator," which consists of tubular coils immersed in an un congealable mixture. A double-acting vacuum-pump volatilizes the agent in the refrigerator coils, and this is attended with the development of an intense cold, which is communicated to the surrounding mixture, and the latter, by means of a circulating pump, is made to flow through a suitable tank containing vessels of water to be frozen, or, if air-cooling only be desired, through iron tubing placed along the walls or ceiling of the chamber to be cooled. The discharge-pipe of the circulating pump communicates with a condenser, which consists of a tubular vessel immersed in a tank containing cooling water taken from any convenient source and kept in constant circulation. The volatilized liquid is expelled from the pump into this condenser, where the process of condensation or liquefaction of the gas is completed. The restored liquid is then returned to the refrigerator by suitable connections, to be again volatilized, and so on continuously, the waste of the agent being but trifling.

The time consumed in the process of freezing the water-cans ranges from twenty-four to thirty-six hours. The more perfect the insulation of the tanks in which the water-cans are immersed, the more quickly is the latent heat extracted from the water; and this, after all, is the problem involved in artificial freezing. To speak of the manufacture of cold, though popularly comprehensible and convenient, is to misapply terms. In one sense heat seems to be but an incident of the cosmic order, an exception to a state of things pervading interstellar space, and toward which the warm earth, and her sister planets, and all the burning orbs of heaven, are gradually tending. In producing cold we therefore seem but to assist Nature to restablish, in an infinitesimal degree, the state of comparative molecular inactivity that distinguishes cold from heat, and which characterizes the vacuum.

The need of an efficient system of artificial refrigeration is constantly increasing. Not alone in warm countries is ice rapidly becoming a universal necessity, but, in myriad industries in temperate climes, the economy experienced by using air-cooling contrivances in the place of Nature's unwieldy, slippery, and not always obtainable product, has long since been satisfactorily demonstrated by the widespread use of various systems of machines.

In the years to come, there may arise some engineering genius bold enough to conceive and skillful enough to execute a plan for tapping the limitless reservoir of cold that pervades interplanetary space, and bringing a supply, regulable at will, to a sweltering world. This would be a highly satisfactory solution of the problem of such interest to nine tenths of humanity for a large portion of the year, how to keep cool. Pending, however, the realization of such a scheme, of which it must be confessed there is no immediate prospect, it is difficult to discern any way to an improvement, in this branch of physics, on the latest product of French inventive genius.